In a conventional mass spectrometer, molecules of a gasified sample are ionized in an ionization chamber, and ions produced there are separated by a mass filter with respect to mass numbers (i.e. ratio of mass (m) to charge (z), m/z). Then, some of the ions pass through the mass filter and enter an ion detector, which generates an electric signal having an intensity corresponding to the number of the ions that has entered. Thus, the intensity of the distribution of the detection signals with respect to mass numbers is obtained.
FIG. 6 shows a configuration of a conventional high accuracy ion detector, coupled with a quadrupole mass filter for separating ions. In the ion detector, an aperture electrode 31 having an opening for letting ions through is disposed at the exit of the quadrupole unit 30 including four rod electrodes. A plate-shaped conversion electrode 32 and an electron multiplier 33 are disposed above and below an incidence axis C of a beam of ions, respectively, opposing to each other across the axis C. The aperture electrode 31 is grounded or an appropriate voltage Va is applied thereto. The conversion electrode 32 has a negative high voltage applied when positive ions are to be detected, or a positive high voltage when negative ions are to be detected.
When, for example, positive ions are to be detected using the above ion detector, the operation is carried out as follows. Ions that have passed through the space defined by the four rod electrodes of the quadrupole unit 30 (only two of them are shown in FIG. 6) along the longitudinal axis C, are converged and pass through the opening of the aperture electrode 31. After that, being attracted by the conversion electrode 32 to which a negative high voltage is applied, the ions travel on upward trajectories and collide on the conversion electrode 32. On the collision of the ions, secondary electrons are emitted from the conversion electrode 32. The secondary electrons travel downward and are captured by the electron multiplier 33. In the electron multiplier 33, the number of the electrons is increased by a repetition of secondary emissions, and a greater number of electrons reach an anode terminal 33a, which is taken out as an electric signal.
When ions having various mass numbers enter the space in the quadrupole unit 30 along the longitudinal axis while a voltage composed of a DC voltage and an AC voltage superposed thereon is applied to the rod electrodes of the quadrupole unit 30, only those ions having a particular mass number corresponding to the voltage is selectively allowed to pass through the space and other ions are diverged. Besides such selected ions, some neutral particles having high energies and other particles also pass through the space in the quadrupole unit 30. These undesired particles may cause a noise in the detection signal if they are captured by the electron multiplier 33. In the above ion detector, however, the neutral particles travel along a straight path in the electric field generated between the conversion electrode 32 and the electron multiplier 33. Thus, noises caused by undesired particles are eliminated and the desired ions can be detected with high accuracy.
In the ion detector, however, the electric field generated between the conversion electrode 32 and the electron multiplier 33 is influenced by the other charged bodies including the aperture electrode 31, so that the distribution of the strength of the electric field is asymmetrical around the central axis extending from the conversion electrode 32 to the electron multiplier 33. Therefore, part of secondary electrons emitted from the conversion electrode 32 fail to travel toward the electron multiplier 33, resulting in a smaller number of electrons to be detected by the electron multiplier 33 and thus deteriorate the efficiency of ion detection.
Furthermore, because of the above-described asymmetry in the electric field, the probability of a secondary electron's reaching the electron multiplier 33 depends on the point where the electron is emitted on the conversion electrode 32. This means that the probability of an ion's being detected depends on the position where the ion passes through the opening of the aperture electrode 31. Accordingly, even when ions of the same mass number pass through the aperture electrode 31 by the same amount, the result of detection may differ depending on the position where the ions pass through the opening of the aperture electrode 31. Because of such an irregularity in the ion detection, the reliability of the mass spectrometer using the above ion detector cannot be very high.